This invention relates to valve gated injection molding, and more particularly to an improved system for molding a product such as a plastic gear or wheel with a hole through it.
As is well known in the art, the hole through the product is formed by the valve pin which extends completely through the cavity and into a bore in the movable mold platen on the other side. Systems using this type of gating which is known as core ring gating are described in Japanese laid-open patent application No. 55061438 of Toyokuni Jushi Kogy and the applicant's U.S. patent application Ser. No. 568,048 filed Jan. 4, 1984 entitled "Injection Molding Core Ring Gate System" which is a continuation-in-part of U.S. patent application Ser. No. 470,192 filed Feb. 28, 1983. As described in the applicant's above mentioned application Ser. No. 470,192, this arrangement facilitates cooling of the molded product by transferring heat along the valve pin to the cool movable mold platen. However, in all of these previous systems, the valve pin has a head portion extending from a reduced neck portion which allows the melt to flow into the cavity around the valve pin. This necessarily has the disadvantage that it requires the valve pin to be operated in the reverse direction to a normal valve gated system. In other words, in these previous core ring gated systems, the valve pin must be actuated between a retracted closed position and a forward open position rather than the reverse with normal valve gated systems.